1. Field of the Invention
This invention relates to artificial joint prostheses and, in particular, to ace tabular prostheses used in hip joint replacements.
2. Description of the Prior Art
Total hip joint replacements comprise a femoral component and an acetabular component. The femoral component is implanted in the patient's femur and the acetabular component is implanted in the acetabular cavity in the patient's pelvis.
The normal acetabular cavity is generally spherically shaped. Accordingly, as illustrated in FIG. 1, during initial hip joint replacement, a spherically-shaped cavity 10 is prepared in the pelvis 14 for the great majority of patients, and the outer surface of the acetabular prosthesis 12 has a generally spherical shape to fit in this cavity. See, for example, Mallory et al., U.S. Pat. Nos. 4,883,491 and 4,944,759.
Over the past two decades, millions of hip joint replacements have been performed. As time has progressed, a significant number of these implants have failed by either or both of the femoral or acetabular components becoming loose. In the case of the acetabulum, a loose prosthesis often erodes bone in the direction of the applied forces during use, i.e., in the superior and posterior directions. This is particularly true where the loosened prosthesis had been implanted using bone cement. As a result, during a second or further repeated hip joint replacement, the surgeon often finds that the once spherical cavity in the pelvis has become elongated in a posterior-superior direction, as shown at 16 in FIG. 2 and 32 in FIG. 23. Also, in some cases, elongated acetabular cavities may be found for patients undergoing their first hip joint replacement.
The classical way to deal with this problem of an elongated acetabular cavity has been to fill a portion of the cavity with a bone graft to create as best as possible a spherical cavity in its normal location. As is evident, this approach suffers from a variety of problems, including availability of bone for the bone graft, mechanical difficulties of securing the bone graft to the host bone, failure of the bone graft to provide and maintain long term mechanical support for the prosthesis, and the hazard of the spread of certain infectious diseases.
In some cases, customized elongated or oblong acetabular prostheses have been used to address this condition. FIGS. 2-5 illustrate this approach to the problem. As shown in these figures, a block of metal has been machined to produce prosthesis 18 having outer surface 20 which is composed of two spherical sections 22 connected by a cylindrical section 24. In these custom prostheses, a cavity 26 has been machined in the body of metal to receive a suitable bearing element 28 (see FIG. 4).
Although these oblong prostheses have represented an improvement over the bone graft approach, they have suffered from a number of problems of their own. First, they have been expensive to manufacture because a separate machining setup has been required for each prothesis geometry which has been produced. As a result, oblong prostheses of this construction have been used in only a limited number of cases.
Even more importantly, the existing oblong prostheses have not fully responded to the anatomical and physiological needs of the patient. This problem is illustrated in FIG. 4. As shown in this figure, the angle in the coronal plane (X-Z plane in FIG. 17) between the face of the bearing element 28 of the prior art oblong prostheses and the transverse plane (X-Y plane in FIG. 17) is typically 60 or more degrees. This orientation results from the contour of the bone and the elongation of the cavity. In medical terminology, the prosthesis is said to have an insufficient amount of adduction, i.e., the angle between the face of the prosthesis and the transverse plane is greater than the preferred angle. (Note that the amount of adduction increases as the angle decreases.)
For joint stability, however, the preferred angle of the face of the bearing element in the coronal plane is on the order of 45 degrees or less. See F. Pipino and P. M. Calderale, "A Biequatorial Acetabular Cup for Hip Prosthesis," Acta Orthopaedica Belgica, Vol. 48, pages 5-13 (1980) and F. Pipino and P. M. Calderale, "A Biequatorial Hip Prosthesis," Panminerva Medica, Vol. 25, pages 231-239 (1983). See also FIG. 19. Thus, the configuration of FIG. 4 is typically more than 15.degree. from the preferred orientation. Moreover, the greater the elongation of the cavity, the greater the departure from the preferred orientation. In terms of function, such a geometry means that the patient will have a significantly higher likelihood of dislocation during use.
In addition to the orientation in the coronal plane, the functionality of the prosthesis is also affected by the orientation of the face of the bearing element in the transverse plane (X-Y plane in FIG. 17). In this case, the preferred angle between the face of the bearing element and the sagittal plane (Y-Z plane in FIG. 17) is on the order of 15.degree. anteverted. See FIG. 20. (Note that the amount of anteversion increases as the angle increases.)
Due to the curvature of the pelvis and the usual direction of erosion and elongation, the prior art oblong acetabular prostheses tend to assume an orientation which is less anteverted or, in some cases, may even be retroverted. The magnitude of this problem also becomes greater as the elongation of the cavity becomes greater. Again, in terms of function, orientations which are less anteverted or are retroverted mean that the patient will have a higher likelihood of dislocation during use.
An alternative construction is shown in FIG. 6 wherein an asymmetric bearing element 30 having a skewed face is used. For a skew which is orientable (see Noiles U.S. Pat. No. 4,678,472), some compensation can be made for the misorientation in both the coronal and transverse planes. FIG. 6 shows a typical improvement in the coronal plane. As shown in this figure, skewing the face of the bearing element can bring the angle in the coronal plane down to around 50.degree.. However, full correction cannot be satisfactorily achieved in this way because a sufficiently large extension of the plastic bearing would be mechanically inadequate to withstand the loads applied during use. Similarly, some improvement in inadequate anteversion can be achieved in the transverse plane, but again full correction cannot be achieved without jeopardizing the mechanical strength of the bearing.